1. Field of the Invention
The present invention relates generally to the fields of cancer research and targeted therapy. More specifically, the present invention relates to fusion constructs comprising an isoform of vascular endothelial growth factor and uses of such constructs.
2. Description of the Related Art
Vascular endothelial growth factor (VEGF)-A plays a central role in the growth and metastasis of solid tumors, and acts as a primary stimulant of vascularization in solid tumors. VEGF-A enhances endothelial cell proliferation, migration, and survival and is essential for blood vessel formation. Other roles of vascular endothelial growth factor include wound healing, vascular permeability and the regulation of blood flow. Through alternative splicing of RNA, human vascular endothelial growth factor exists as at least four isoforms of 121, 165, 189, or 206 amino acids. The lowest molecular weight isoform, designated VEGF121, is a non-heparan sulfate-binding isoform that exists in solution as a disulfide-linked homodimer.
VEGF is released by a variety of tumor cells. The angiogenic actions of VEGF are mediated through two related receptor tyrosine kinases, kinase domain receptor (KDR) and FLT-1 in the human, and Flk-1 and Flt-1 in the mouse. Both are largely restricted to vascular endothelial cells. KDR/FIk-1 and FLT-1 receptors are overexpressed on the endothelium of tumor vasculature. In contrast, these receptors are almost undetectable in the vascular endothelium of adjacent normal tissues. The receptors for vascular endothelial growth factor thus seem to be excellent targets for the development of therapeutic agents that inhibit tumor growth and metastatic spread through inhibition of tumor neovascularization.
To this end, VEGF121 would be an appropriate carrier to deliver a toxic agent selectively to tumor vascular endothelium. VEGF121, exists in solution as a disulfide linked homodimer and binds to KDR and FLT-1 in a heparin-independent manner. It does not bind neuropilin-1 or neuropilin-2. VEGF121 has been shown to contain the full biological activity of the larger variants.
Molecular engineering enabled the synthesis of novel chimeric molecules having therapeutic potential. Chimeric fusion constructs targeting the IL-2 receptor, the EGF receptor, and other growth factor/cytokine receptors have been described. It has also been showed that a chemical conjugate of vascular endothelial growth factor and truncated diphtheria toxin has impressive cytotoxic activity on cell lines expressing receptors for vascular endothelial growth factor. Further studies with VEGF/diphtheria toxin fusion constructs demonstrated selective toxicity to Caprice's sarcoma cells and dividing endothelial cells in vitro and in vivo. However, the prior art is deficient in fusion constructs comprising vascular endothelial growth factor and other cytotoxic molecule with improved biochemical and pharmacological properties. The present invention fulfills this long-standing need and desire in the art.